Debra Lieberman

Current Research Interests

The central goal of my research is to understand how evolution has shaped the social mind. To this end I apply theoretical tools from evolutionary biology to develop hypotheses regarding function, then generate information-processing models that specify how the functional mechanism operates, and then empirically test the validity of these models. I study a range of phenomena including kinship, altruism, sexuality, disgust, morality, and, more recently, gratitude. Below I provide a brief summary of three strands of my ongoing work.

1. How do humans learn who counts as a close genetic relative?

During the 1950s and 60s, the modern synthesis in biology brought into focus the role that genes play in the process of natural selection, thus Mendelizing Darwinian views of the evolutionary process. One of the major advancements during this time period was the development and publication of inclusive fitness theory (Hamilton, 1964), which recognized that adaptations for altruism could evolve between kin. Kin are individuals with an increased probability above population average of sharing the same genes by virtue of common descent. Critically, the mathematical equation modeling when it would pay for an individual to behave altruistically (C<rB) posited a variable, r, which captures the degree of relatedness between two individuals. Biologists soon realized that somehow, individual organisms were computing r to regulate altruistic as well as inbreeding avoidance behaviors. In the decades following the publication of inclusive fitness theory, biologists documented how non-human species ranging from bees to lions performed this process of "kin detection" (Hepper, 1991). My research addresses this fundamental issue in humans and investigates how, throughout development, humans learn to distinguish kin from non-kin, and how kin detection regulates altruistic and sexual motivations. In a series of publications (e.g., Lieberman, Tooby, & Cosmides, 2003, Proceedings B; Lieberman, Tooby, & Cosmides, 2007, Nature; Lieberman & Lobel, 2012, Evolution and Human Behavior), my co-authors and I discuss the kinship cues that evolved to regulate sexual and altruistic motivations toward siblings.

2. What are the evolved functions of disgust?

Researchers across a wide range of disciplines have become interested in the emotion disgust. Many of these researchers utilize the model of disgust put forth by Paul Rozin, Jonathan Haidt, and colleagues (Rozin et al., 2008). According to this model, there are 4 different disgust domains: core disgust, animal reminder disgust, interpersonal disgust, and moral disgust. Though this model incorporates aspects of evolved function, it falls short of providing a rigorous explanation of why we find particular behaviors and items disgusting as well as the selection pressures that led to the evolution of disgust adaptations. In an attempt to revise the current understanding of disgust, I developed a new model of disgust with my colleague Josh Tybur. We proposed that disgust evolved to regulate the avoidance of substances harboring pathogens and was co-opted to regulate decisions regarding mating behavior (e.g., incest avoidance) and, ultimately, other social transgressions (Tybur, Lieberman, & Griskevicius, 2009, Journal of Personality and Social Psychology). My colleagues and I have found behavioral and neuroscientific evidence that disgust does indeed partition into these three functional domains (Schaich Borg, Lieberman, & Kiehl, 2008, Journal of Cognitive Neuroscience). We have used this theoretical framework to develop a scale measuring individual differences in disgust sensitivities, the Three Domain Disgust Scale (Tybur et al., 2009), and have developed a computational model of disgust that specifies the types of information processing systems required to perform each domain-specific function (Tybur, Lieberman, Kurzban, & DeScioli, 2013, Psychological Review).

3. What is the evolved function and computational structure of gratitude?

Biologists have developed robust theories to explain why lifelong associations occur among close genetic relatives and within social exchange relationships, but human prosociality extends beyond kin and trading partners to individuals who tend not to be related and with whom direct tit-for-tat reciprocity rarely occurs. In short, humans have friends. Despite decades of research on the factors that drive benefit delivery between family members and reciprocal exchange partners, far less work has examined the systems that motivate the development and maintenance of friendships. My research asks the question of whether gratitude is a key emotion that evolved to initiate and maintain friendships. Together with Dr. Michael McCullough, and grad students Eric Pedersen, Adam Smith, and Daniel Forster, I am developing a computational model of gratitude that shows how this emotion might be the key to explaining why and how humans are a social species. We have received a grant via the Berkeley Greater Good Science Center to explore this possibility.

Lieberman, D., Oum, R. E., & Kurzban, R. (2008). The family of fundamental social categories includes kinship: Evidence from the memory confusion paradigm. European Journal of Social Psychology, 38, 998-1012.